Biomass (material derived from living or recently living biological materials) is becoming one of the most important renewable energy resources. The ability to convert biomass to fuels, chemicals, energy and other materials is expected to strengthen rural economies, decrease dependence on oil and gas resources, and reduce air and water pollution. The generation of energy and chemicals from renewable resources such as biomass also reduces the net rate of carbon dioxide production, an important greenhouse gas that contributes to global warming.
A key challenge for promoting and sustaining the use of biomass in the industrial sector is the need to develop efficient and environmentally benign technologies for converting biomass to useful products. Present biomass conversion technologies unfortunately tend to carry additional costs which make it difficult to compete with products produced through the use of traditional resources, such as fossil fuels. Such costs often include capital expenditures on equipment and processing systems capable of sustaining extreme temperatures and high pressures, and the necessary operating costs of heating fuels and reaction products, such as fermentation organisms, enzymatic materials, catalysts and other reaction chemicals.
One alternative fuel technology receiving significant attention is biodiesel produced via the esterification of vegetable oils or animal fats. The US production of biodiesel is reaching 30-40 million gallons annually, but is projected to grow to a targeted 400 million gallons of production per year by 2012. In Europe, over 1.4 metric tons of biodiesel was produced in 2003, and major initiatives are underway in Brazil and Asia.
A byproduct of the biodiesel process is crude glycerol, which has little or no value without further refinement. The issue is what to do with the escalating supply of crude glycerol. Purification of crude glycerol is one option, however, the refining of crude glycerol, which contains catalyst, organic impurities and residual methanol, is difficult and often too expensive for small scale biodiesel producers. To complicate matters, the demand for pure glycerol has also remained static and prices have dropped dramatically as more supply is brought on line, especially in Europe.
The development of effective methods to convert crude glycerol to alternative products, such as diols and other polyols, ketones, aldehydes, carboxylic acids and alcohols, may provide additional opportunities to improve the cost effectiveness and environmental benefits of biodiesel production. For example, over a billion pounds of propylene glycol is produced in the United States today and used in the manufacture of many industrial products and consumer products, including aircraft and runway deicing fluids, antifreeze, coolants, heat transfer fluids, solvents, flavors and fragrances, cosmetic additives, pharmaceuticals, hydraulic fluids, chemical intermediates, and in thermoset plastics. Propylene glycol is currently produced via the partial oxidation of fossil fuel derived propylene to form propylene oxide, which is then reacted with water to form propylene glycol.
Researchers have recently developed methods to react pure hydrogen with larger biomass-derived polyols (glycerol, xylitol, and sorbitol) and sugars (xylose and glucose) over hydrogenation and hydrogenolysis catalytic materials to generate propylene glycol. While the biomass is derived from a renewable source, the pure hydrogen itself is generally derived through the steam reforming of non-renewable natural gas. Due to its origin, the pure hydrogen must also be transported to and introduced into the production stream at elevated pressures from an external source, thereby decreasing the efficiency of the process and causing an increase in the overall cost of the ultimate end-product.
For instance, U.S. Pat. Nos. 6,841,085, 6,677,385 and 6,479,713 to Werpy et al., disclose methods for the hydrogenolysis of both carbon-oxygen and carbon-carbon bonds using a rhenium (Re)-containing multimetallic catalyst in the presence of external hydrogen to produce products such as propylene glycol (PG). The Re-containing catalyst may also include Ni, Pd, Ru, Co, Ag, Au, Rh, Pt, Ir, Os and Cu. The conversion takes place at temperatures in a range from 140° C. to 250° C., and more preferably 170° C. to 220° C., and a hydrogen pressure between 600 psi to 1600 psi hydrogen.
Dasari et al. also disclose hydrogenolysis of glycerol to PG in the presence of hydrogen from an external source, at temperatures in a range from 150° C. to 260° C. and a hydrogen pressure of 200 psi, over nickel, palladium, platinum, copper and copper-chromite catalysts. The authors reported increased yields of propylene glycol with decreasing water concentrations, and decreasing PG selectivity at temperatures above 200° C. and hydrogen pressures of 200 psi. The authors further reported that nickel, ruthenium and palladium were not very effective for hydrogenating glycerol. Dasari, M. A.; Kiatsimkul, P.-P.; Sutterlin, W. R.; Suppes, G. J. Low-pressure hydrogenolysis of glycerol to propylene glycol Applied Catalysis, A: General, 281(1-2), p. 225 (2005).
U.S. patent application Ser. No. 11/088,603 (Pub. No. US2005/0244312 A1) to Suppes et al., disclose a process for converting glycerin into lower alcohols having boiling points less than 200° C., at high yields. The process involves the conversion of natural glycerin to propylene glycol through an acetol intermediate at temperatures from 150° C. to 250° C., at a pressure ranging from 1 to 25 bar (14.5 to 363 psi), and preferably from 5 to 8 bar (72.5 to 116 psi), over a palladium, nickel, rhodium, zinc, copper, or chromium catalyst. The reaction occurs in the presence or absence of hydrogen, with the hydrogen provided by an external source. The glycerin is reacted in solution containing 50% or less by weight water, and preferably only 5% to 15% water by weight.